Problem: Which of these numbers is prime? ${27,\ 29,\ 51,\ 60,\ 65}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 27 are 1, 3, 9, and 27. The factors of 29 are 1 and 29. The factors of 51 are 1, 3, 17, and 51. The factors of 60 are 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and 60. The factors of 65 are 1, 5, 13, and 65. Thus, 29 is a prime number.